<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* display: inline-flex;行内块级 */
        /* container  item */
        
        .box {
            display: flex;
        }
    </style>
</head>

<body>
    <!-- 在父元素中垂直居中一个块内容  定位 top 0 bottom 0
上下margin auto 

    -->
    aaa
    <div class="box">
        <div class="item">1</div>
        <div class="item">2</div>
        <div class="item">3</div>
    </div>
    bbb
</body>

</html>